Cape Town - Phil Jones and Fabian Delph were absent from England training on Monday, casting doubt on their availability against Slovenia on Thursday.
Jones has impressed for Manchester United this season, their defence shipping just two goals in seven Premier League outings to sit joint-top at the summit with neighbours Manchester City.
Delph also caught the eye during the early stages of the new campaign, filling in for City manager Pep Guardiola where needed, with the Spaniard praising his versatility last week.
But the pair were due to be assessed by the Three Lions medical team on Monday afternoon for unspecified knocks, having not linked up with the other 23 members put through their paces by manager Gareth Southgate at St George's Park.
Southgate's side lead Group F in World Cup qualifying by five points ahead of Slovakia with two games remaining, and victory over the Slovenians at Wembley this week will secure their passage to Russia 2018.